Many integrated optical circuits exist, consisted of a planar substrate and an optical waveguide deposited on the planar substrate. The IOCs are generally manufactured by techniques of microlithography and/or thermal diffusion. The integrated optical circuits may be manufactured from substrates of different materials such as semiconductors, glass or lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). There exist different optical components on integrated optical circuit: polarizer, phase modulator, Mach-Zehnder interferometer, Y-junction, 2×2 coupler or 3×3 coupler. Especially, the integrated optical circuits formed by proton exchange on lithium niobate find many applications, for example in rotation measurement systems.
By comparison with the optical devices in free space, optical devices on integrated optical circuit offer the advantage to allow the integration of several functions on a same IOC, which allows improving the compactness and reducing the optical connections.
However, the specifications of the optical devices on integrated optical circuit have certain limitations. Hence, a waveguide polarizer on integrated optical circuit has a polarization extinction ratio that is limited to about −45 to −65 dB.
FIG. 1 schematically shows a perspective view of a waveguide polarizing device according to the prior art and illustrates the operation of this device.
The polarizing device 40 comprises an integrated optical circuit comprising a planar substrate 10 and a polarizing waveguide 6. By convention in the present disclosure, the substrate 10 comprises an input face 1, an output face 2, a lower face 4, an upper face 3 and two lateral faces. The lower face 4 and the upper face 3 extend between the input face 1 and the output face 2. The lower face 4 and the upper face 3 are opposed to each other. Preferably, the lower face 4 and the upper face 3 are planar and parallel to each other. Likewise, the lateral faces are planar and parallel to each other and extend between the input face 1 and the output face 2. The optical waveguide 6, for example rectilinear, extends between a first end 7 on the input face 1 and a second end 8 on the output face 2. The input 1 and output 2 faces of the substrate may also be planar and polished, but are preferably cut with an inclination angle with respect to the axis of the waveguide at the ends 7 and 8 to avoid the spurious back-reflections. By convention, the waveguide 6 is closer to the upper face 3 than to the lower face 4. Such a waveguide polarizer 6 on an integrated optical circuit may be easily connected by sections of optical fiber 20, 30 to other optical components such as a light source or a detector.
In the case of a lithium niobate proton-exchange polarizer, the optical waveguide 6 is located under the upper face 3 of the substrate and extends in a plane parallel to the upper face 3. The optical waveguide 6 forms a waveguide polarizer that guides only one polarization. The optical waveguide 6 may be delimited by the upper face or be buried just under this upper face. In other types of IOC, the waveguide 6 may be deposited on the upper surface 3 or may extend inside the substrate, for example in a plane parallel to the upper surface 3, at half the distance between the lower face 4 and the upper face 3. An input optical fiber 20 and an output optical fiber 30 are optically coupled to the first end 7 and to the second end 8, respectively, of the waveguide 6.
The input optical fiber 20 transmits an optical beam in the integrated optical circuit. A part of the optical beam is guided by the waveguide. The guided beam 12 propagates up to the end 8 of the waveguide 6 opposite the output fiber 30. Due to a mode mismatch between the core of the optical fiber 20 and the integrated waveguide 6, another part of the beam is not coupled in the waveguide and propagates freely in the substrate 10. A non-guided beam 14 then propagates in the substrate up to the lower face 4 of the substrate. A part of the non-guided beam 14 may be reflected by total internal reflection on the lower face 4. A part of the reflected beam 16 may then be transmitted up to the end of the substrate opposite the output fiber 30. The output fiber 30 may hence collect not only the guided optical beam 12, but also a part of the non-guided and reflected optical beam 16. An input beam is generally coupled to an end of an optical waveguide through an optical fiber. However, only certain modes, for example of polarization, are guided by the waveguide, the other modes propagating freely in the substrate. Moreover, it the core of the fiber is not perfectly aligned with the waveguide of the integrated optical circuit, a part of the incident light beam may be coupled in the substrate and may propagate outside the waveguide. A part of the light non-guided by the waveguide may be reflected by total internal reflection on one or several faces of the substrate. Finally, a part of this non-guided light may be coupled to an output optical fiber opposite another end of the waveguide. The non-guided light may hence disturb the operation of an integrated optical circuit. For example, in the case of a lithium niobate proton-exchange polarizer, the polarization rejection ratio may be affected by the coupling of the light transmitted by the substrate in a non-guided manner.
FIG. 1 shows only a single reflection on the lower face 4 of the substrate, at half the distance between the input face 1 and the output face 2, i.e. at the centre of the lower face 4. However, other multiple internal reflections are also possible. Indeed, the substrate may transmit different non-guided beams propagating by internal reflection, in particular on the lower face 4, but also by multiple reflections between the lower face 4 and the upper face 3, or on the lateral faces 5. Spurious non-guided beams propagating by internal reflection on the faces of the substrate may arrive near the waveguide end 8 of the output face 2 of the substrate.
Generally, the non-guided beams reflected inside the substrate may affect the quality of the signals transmitted in the waveguide of an integrated optical circuit. In the case of a lithium niobate proton-exchange polarizer, cut along an X plane and comprising an integrated waveguide along the Y axis of propagation, the guided beam 12 is generally a beam of TE polarization and the non-guided beam 14 is a beam of TM polarization. Due to the internal reflections of non-guided light in the substrate, the polarization rejection ratio of a proton-exchange polarizer according to the scheme of FIG. 1 is in practice limited to about −50 dB.
Now, the quality of a polarizer influences the performances of certain applications. It is hence necessary to improve the rejection ratio of an integrated waveguide polarizer.
Different solutions have been proposed to solve the problem of spurious coupling of non-guided optical beam between a waveguide input and a waveguide output in an integrated optical circuit.
It is generally admitted that the main contribution to the spurious light comes from the primary reflection of a non-guided beam 14 at a primary reflection point located at the centre of the lower face 4 between a first waveguide end 7 on the input face 1 and a second waveguide end 8 on the output face 2.
In order to suppress the primary reflection on the lower face of a substrate 4, it has been proposed to machine a central groove arranged in the middle of the lower face 4 and extending transversally to the optical waveguide 6. A central groove on the lower face of the substrate allows absorbing or deviating the beams propagating in the substrate and hence improving the rejection ratio of a proton-exchange polarizer by several orders of magnitude. However, in practice, the rejection ratio of a proton-exchange polarizer remains limited to about −65 dB.
The invention aims to propose an alternative or complementary solution to the formation of a central absorbing groove on the lower face of a polarizing waveguide integrated optical circuit to increase the rejection ratio of a waveguide polarizing device.
FIG. 2 schematically shows a sectional view of a detail of the polarizing device 40 of FIG. 1. The end of the optical fiber 30 is connected to a waveguide polarizer 5 formed on an integrated optical circuit substrate 10. The optical fiber 30 has a preferably single-mode core 31. The end of the optical fiber 30 is generally bonded to a ferrule 32, which allows connecting and aligning the end of the optical fiber 30 to the waveguide 6. The core 31 of the optical fiber is aligned and centred with respect to the waveguide 6 formed, for example, by proton exchange on a lithium niobate substrate 10. The optical fiber 30, via a ferrule, is made integral with the integrated optical circuit by means of glue 5 that is transparent at the wavelength used.
The mounting of FIG. 2 advantageously allows combining a polarization filter by means of the waveguide polarizer 6 and a spatial single-mode filter by means of the fiber 30.
The waveguide polarizer 6 formed by proton exchange allows separating, on the one hand, a polarization state, for example TM, guided in the waveguide 6, and on the other hand, a polarization state, for example TM, non-guided, that propagates in the substrate 10. The waveguide polarizers integrated on a lithium niobate substrate have a very high polarization rejection ratio for the non-guided polarization and a very limited insertion loss for the guided polarization.
However, as detailed in connection with FIG. 1, the waveguide polarizer 6 operates by an effect of selective guiding in polarization and not by absorption. That way, a part of the non-guided beam that propagates in the substrate 10 may be recoupled in the optical fiber at the output of the IOC after one or several internal reflections in the substrate.
In practice, in a proton-exchange polarizer 6 as illustrated in FIG. 2, the internal reflections of non-guided light in the substrate limit the polarization rejection ratio to a power of about −45 dB. Integrated circuits including a groove in the rear face of the substrate allow attenuating the spurious internal reflections and obtaining a polarization rejection ratio of at best −65 dB.
It results therefrom that a residual component of TM polarization that propagates via the substrate 10 of a proton-exchange polarizer 6 can be transmitted via the optical fiber 30 to an optical measurement system, for example for rotation measurement. Now, a so-called single-mode optical fiber 30 supports in reality two polarization modes.
The quality of the polarizer influences the performances of certain applications.
Searching to avoid the propagation of the TM polarization beam in the substrate of a waveguide polarizer, it has been proposed to place another polarizer (such as, for example, a polarization splitter cube, a crystal plate) for example upstream of the input fiber 20 and to connect this other polarizer to the integrated optical circuit via a polarization-maintaining fiber.
Another solution consists in using a polarizing fiber on the input fiber path 20. However, a polarizing fiber has for drawback to be very sensitive to the curvatures: the losses and the polarization extinction ratio (PER) are modified as a function of the radius of curvature and of the axis of such a curvature.